Large outflow channels on ancient terrains of Mars have been interpreted as the products of catastrophic flood events. The rapid burial of water-rich sediments following such flooding could have led to sedimentary volcanism, in which mixtures of sediment and water (mud) erupt to the surface. Tens of thousands of volcano-like landforms populate the northern lowlands and other local sedimentary depocenters on Mars. However, it is difficult to determine whether the edifices are related to igneous or mud extrusions, partly because the behaviour of extruded mud under martian surface conditions is poorly constrained. Here, we investigate the mechanisms of mud propagation on Mars using experiments performed inside a low-pressure chamber at cold temperatures. We find that low viscosity mud under martian conditions propagates differently from on Earth, because of rapid freezing and the formation of an icy crust. Instead, the experimental mud flows propagate like terrestrial pahoehoe lava flows, with liquid mud spilling from ruptures in the frozen crust, then refreezing to form a new flow lobe. We suggest that mud volcanism can explain the formation of some lava-like flow morphologies on Mars, and that similar processes may apply to cryovolcanic extrusions on icy bodies in the Solar System. The physics behind igneous volcanism on Mars is better understood [e.g., 1-4] than that of sedimentary volcanism in which mixtures of water and sediment, subsequently referred to as mud, are extruded onto the surface. On Earth, sedimentary volcanism manifests at the surface as eruptions of fluids (water, gas, occasionally oil), fine grained sediments (e.g. clays) and clasts from the country-rock. These geological phenomena are the result of fluid (on Earth typically associated with methane) overpressure [5], generated at several hundred to several thousand metres depth, combined with gravitational instability of buoyant sedimentary units buried at deeper stratigraphic levels [6]. The viscosity of ascending mud varies, and affects the shapes, sizes and thicknesses of resulting flows. The higher the water

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Sediment mobilisation occurring at depth and ultimately manifesting at the surface, is a process which may have operated on Mars. However, the propagation behaviour of this mixture of water and sediments (hereafter simply referred to as mud) over the martian surface, remains uncertain. Although most of the martian surface is below freezing today, locally warmer surface temperatures do occur, and our current knowledge suggests that similar conditions prevailed in the recent past. Here, we present the results of experiments performed inside a low pressure chamber to investigate mud propagation over a warm (~295 K) unconsolidated sand surface under martian atmospheric pressure conditions (~7 mbar). Results show that the mud boils while flowing over the warm surface. The gas released during this process can displace the underlying sand particles and hence erode part of the substrate. This "entrenched" flow can act as a platform for further mud propagation over the surface. The escaping gas causes intermittent levitation of the mud resulting in enhanced flow rates. The mud flow morphologies produced by these phenomena differ from those produced when mud flows over a frozen martian surface as well as from their terrestrial counterparts. The intense boiling removes the latent heat both from the mud and the subsurface, meaning that the mud flow would eventually start to freeze and hence changing again the way it propagates. The diverse morphology expressed by our experimental mudflows implies that caution should be exercised when interpreting flow features on the surface of Mars and other celestial bodies.

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